Glaucoma is a leading cause of blindness in the US, characterized by a gradual loss of vision due to damage to optic nerve tissue. The ganglion cell layer (GCL) of the retina contains the cell bodies of the retinal nerve fibers, which travel through the retinal nerve fiber layer (RNFL) and coalesce at the optic nerve head (ONH) to form the optic nerve. One hallmark of glaucoma is the progressive loss of neural tissue in all these structures and consequent visual loss.
Glaucoma usually responds well to treatment, but treatment cannot restore lost neural tissue. It is thus important to diagnose the disease quickly to prevent irreversible visual loss. Glaucoma diagnostic methods can generally be classified as functional or structural. Functional tests measure how the disease is affecting the patient's visual functioning. Perimetry measurements, which indicate directly the impairment to a patient's visual field, are the current diagnostic standard. However, visual field changes may not be detectable until significant damage has been done to the RNFL. In some cases, up to fifty percent of nerve fibers can be damaged before a visual field defect can be detected by perimetry. Thus structural diagnostic techniques are likely to be more successful in glaucoma screening and in the management of early glaucoma, when irreversible visual loss can be prevented.
Structural diagnostic methods measure the loss of neural tissue due to glaucoma. Several techniques are currently available for estimating the retinal nerve fiber layer in clinical practice. Indirect ophthalmoscopy and fundus photography can be used to provide a qualitative assessment of the retinal never fiber layer. In contrast, objective methods such as confocal scanning laser ophthalmoscopy, scanning laser polarimetry, and optical scanning tomography have been developed to quantify retinal nerve fiber layer thickness across the posterior pole.